An internet protocol (IP) multimedia core network subsystem (IMS) is an IP-based network architecture put forward by the 3rd generation partnership project (3GPP), and establishes an open and flexible service environment. In an IMS service system, the control layer and the service layer are separated; and the control layer takes the responsibility of providing necessary functions such as triggering, routing, and charging to the service layer. Service triggering and controlling functions in the control layer are carried out by a call session control function (CSCF, wherein the CSCF is divided into three types: proxy, interrogating, and serving, wherein the serving CSCF (S-CSCF) takes primary responsibilities). The service layer comprises a series of application servers (AS), and provides specific service servings. The AS can be either an independent entity or an entity in the S-CSCF. The control layer controls the service triggering according to subscription information of users, invokes services on the AS, and implements service functions.
User equipments (UE) can access an IMS network through a packet switching (PS) domain of a 2G/3G network, can also access an IMS network through an evolved packet system (EPS) network which only consists of the PS domain, and can also access an IMS network through a circuit switching (CS) domain of a 2G/3G network and a media gateway control function (MGCF, which can implement a conversion between CS domain signalings and IMS network signalings) of the IMS network. A schematic architectural diagram of that a UE accesses an IMS network via the above various ways to implement the services is as shown in FIG. 1. When the UE is in an area covered by both the 2G/3G network and the EPS network, the UE can determine to access, according to circumstances, the PS domain of the EPS network, the CS domain of the EPS network, or the 2G/3G network.
In the prior art, considering that not all the network areas covered by the PS domain of the 2G/3G network and/or the EPS network support IMS voice over PS session (IMS VOIP) during network deployment especially during network evolvement process, the 2G/3G network takes a routing area (RA) as granularity to distinguish IMS VOIP capability, and a serving CPRS support node (SGSN) knows the IMS VOIP capability of the UE in one routing area; and the EPS network takes a tracking area (TA) list as granularity to distinguish the IMS VOIP capability, and a mobility management entity (MME) knows the IMS VOIP capability of the UE in one tracking area.
The SGSN is a mobility management network element of the PS domain of the 2G/3G network; and the MME is a mobility management network element of the PS domain of the EPS network. Under a circumstance shown in FIG. 1, both the MME and the SGSN are independent network elements. During practical application and network deployment, both the MME and the SGSN can be a combo node (which is also called as composite node or combined node, i.e., combined MME/SGSN). The MME and the SGSN have similar functions, and both of the MME and the SGSN are the mobility management network elements of the PS domain, so a special node which integrates functions of both the SGSN and the MME may exist during a network evolvement process. The interface between the SGSN and the MME is an internal interface. The message communications between the MME and the SGSN comprised in the combo nodes are performed by way of internal messages or sharing internal memory. The combined node has simple network deployment, can reduce signaling intercommunication time delay among network elements, and may be used by some operators when the EPS system is deployed. When the network is deployed by using the combo node, if the UE initiates registration in a wireless access system and selects the combo node, when the UE moves to another wireless access system later, in the prior art, the combined node will be selected for the UE under a situation that the serving range is available.
As mentioned above, the UE can access the IMS to perform a session by above multiple ways. During a terminated call, in order to route the call to the UE effectively, the IMS network needs to perform the domain selection. In the related art, in order to improve the effect of domain selection during the terminated call, an implementation method is a method for domain selection on the basis of information such as the IMS VOIP capability provided by a home subscriber server (HSS). The concept of the method is as follows: when the UE performs attach/update process, the SGSN/MME notifies the HSS whether an area currently being accessed by the UE supports the IMS VOIP by an update location message; and afterwards when the VOIP in the area to which the UE accesses is changed, the SGSN/MME also needs to send a message to notify the HSS. During the terminated call, the AS queries the HSS to obtain the related information such as the IMS VOIP, so as to determine how to perform the domain selection.
The method for implementing the domain selection during the terminated call in the related art will be described hereinafter in connection with FIG. 2 by taking a circumstance that the UE performs the attach/update at the SGSN in the 2G/3G network at first and then moves to the EPS network to perform update at the MME as an example. The method and flow that the UE performs the attach/update at the MME in the EPS network at first and then moves to the 2G/3G network to perform update at the SGSN is similar to the above method. FIG. 2 is a schematic flowchart of a method for implementing domain selection during a terminated call in the related art, and the method comprises the steps as follows.
Assuming that a UE powers on or performs attaching by inserting an SIM card in it under an SGSN, or the UE moves to an area covered by the SGSN to update a routing area.
Steps 2A1 to 2A8 are a process of a related method that the SGSN notifies the HSS of the IMS VOIP capability of the network area (i.e., the routing area) currently being accessed by the UE during the attach/update process performed by the UE in the 2G/3G network.
Step 2A1, the UE performs the attach/routing area update in the 2G/3G network;
Step 2A2, user context does not exist in the SGSN; and the SGSN sends an update location message to the HSS, wherein the update location message carries an IMS VOIP indicator which indicates whether the network area in which the UE is currently located supports the IMS VOIP.
Step 2A3, the HSS interacts with the SGSN, so as to respond with an update location response and insert subscriber data.
Step 2A4, the SGSN sends an attach/routing area update accept message to the UE to complete the attach/update process.
When the UE moves to a new routing area, the following steps will be performed.
Step 2A5, the UE performs the routing area update, and sends a routing area update message to the SGSN.
Step 2A6, the SGSN receives the routing area update message. At this time, the SGSN needs to judge whether the IMS VOIP capability of the area currently being accessed by the UE is changed. If the IMS VOIP capability of the routing area currently being accessed by the UE has changed, the SGSN needs to execute step 2A7 to notify the HSS that the IMS VOIP capability of the routing area currently being accessed by the UE is changed; otherwise, step 2A7 will not be executed.
Step 2A7, the SGSN sends a message to the HSS to notify the HSS of the IMS VOIP capability of the routing area currently being accessed by the UE.
Step 2A8, the SGSN sends a routing area update accept message to the UE.
Steps 2B1 to 2B9 are a process of the related method that the MME notifies the IMS VOIP capability of the network area (i.e., a tracking area list) being accessed by the UE currently to the HSS during the update process performed by the UE in the EPS network.
The UE moves to the EPS network access.
Step 2B1, the UE initiates a tracking area update process, and sends a tracking area update message to the MME.
Step 2B2, the MME finds the SGSN where the UE originally registered according to the information carried in the message sent by the UE, and obtains the UE context from the SGSN. In the process, the MME may activate an idle mode signaling redundancy (ISR) function; and an ISR correlation is established between the SGSN and the MME.
Step 2B3, since user context does not exist in the MME, the MME sends an update location message to the HSS, wherein the message carries the IMS VOIP capability of the network area accessed by the UE currently.
Step 2B4, subscriber data insertion and an update location response process are performed between the HSS and the MME.
Step 2B5, the MME sends a tracking area update accept message to the UE.
The UE moves into a tracking area which is not in the registered tracking area list. Since the tracking area is not updated, the UE initiates the tracking area update.
Step 2B6, the UE sends a tracking area update message to the MME.
Step 2B7, the MME judges whether the IMS VOIP capability of the network area currently being accessed by the UE is changed, wherein if the IMS VOIP capability is changed, the MME needs to execute step 2B8; otherwise, the step 2B8 is unnecessary to execute.
Step 2B8, the MME sends a message to the HSS to notify the IMS VOIP capability of the network area where the UE is currently located to the HSS.
Step 2B9, the MME sends a tracking area update accept message to the UE.
Subsequently, the UE may move, power on or power off in the network to perform an attach/update process similar to the above attach/update process, which is very easily known by those skilled in the art and will not be redundantly described here. Then the process continues until the CSCF/AS receives a request that session is required for the UE and the domain selection needs to be performed. The following steps 2C1 to 2C4 are the process of the domain selection after a CSCF/AS receives a call session request.
Step 2C1, the CSCF/AS receives the terminated session request to the UE.
Step 2C2, the AS sends a query request message to the HSS, so as to query information such as the IMS VOIP capability of the network area recently accessed by the UE.
Step 2C3, the HSS returns a query response message to the AS, wherein the message carries the IMS VOIP capability of the network area recently accessed by the UE.
Step 2C4, the AS performs the domain selection during the terminated call according to the information provided by the HSS.
After the above-mentioned method for selecting the domain during the terminated call is introduced, under the circumstance that the UE has already registered in the SGSN and the MME, when the IMS VOIP capability of the routing area and the tracking area accessed by the UE changes, the SGSN and the MME are also required to send signalings to the HSS, incurring the signaling loads between the SGSN/MME and the HSS. In practical applications, most users may move in the network; and the moving area includes different routing areas and tracking areas which support the IMS VOIP capability. However, the users may rarely receive calls, and thus during the calls the CSCF/AS needs to send the calls to the UE and the AS needs to perform the domain selection during the terminated calls. Therefore, comparing with the improvement on the accuracy of the domain selection brought by using the technology, the newly added signalings between the SGSN/MME and the HSS result in that the advantageous effects brought by the technology are extraordinary limited.
Assuming that a network is deployed as shown in FIG. 3 and the network area has a traffic vehicle (e.g. subway) which can simultaneously carry a large number of users, and assuming that the method in the above related art is applied in this case, during morning and evening rush hours everyday, since a large number of users move from one place to another place, even though few users have incoming calls, a great deal of signalings still exist between the MME/SGSN and the HSS in order to carry out the domain selection during the terminated calls in the related art, which will bring certain risks to the stable operation of the network, and may also affect the users to complete some services. In order to completely avoid the above situation, various requirements and limitations are put forward to the network deployment.
When the method for selecting the domain during the terminated call in the related art is applied under the circumstance that the ISR is activated, the situation becomes more complicated. The ISR is a function supported by the 3GPP (3rd Generation Partnership Project) for the purpose of reducing the network signaling loads brought by when the UE is in the idle state and in an area covered by both the 2G/3G network and the EPS network, the UE repeatedly selecting the 2G/3G network and the EPS network and initiating registrations. The main concept is that: when the UE registers in both the SGSN and the MME at the same time, both the SGSN and the MME save the context information of the user, and registration information of the two networks is also saved in the UE, in this way, the UE will not perform TAU (Tracking Area Update) or RAU(Routing Area Update) processes any more when later the UE needs to select between the two networks, unless the routing area in which the UE is currently located is not registered or the tracking area in which the UE is currently located is not in the registered tracking area list. Under the circumstance that the ISR is activated, since the UE does not initiate the update when the system accessed by the UE changes, the network can not know which access system the UE is currently in when the UE is in the idle state.
The above area with the ISR function and being covered by both the 2G/3G network and the EPS is also called an ISR area. During the network deployment, the ISR area may be the same as the area which supports the IMS VOIP, and may be different from the area which supports the IMS VOIP. If the ISR is the same as the area which supports the IMS VOIP, the method is the same as the method shown in FIG. 2; and if the ISR is different from the area which supports the IMS VOIP, the method has some other requirements on the basis of the method shown in FIG. 2.
For example, as shown in FIG. 4, an overlapped coverage area of the EPS network and the 2G/3G network supports the ISR function (i.e., the ISR area); but in the ISR area, the UE supports the IMS VOIP by access through the EPS network, and the UE does not support the IMS VOIP by access through the 2G/3G network or not all the 2G/3G areas support the IMS VOIP. Assuming that the UE performs the attach/update through RA1 access which does not support the IMS VOIP, during the update process, the SGSN notifies the HSS that the IMS VOIP is not supported; then the UE moves to the EPS access and performs the TAU process. Assuming that the ISR is activated during the update process, the UE may move back to the RA1 later and does not initiate the update process, and the terminated call is required to be performed through the CS domain when the UE accesses through the RA1 of the 2G/3G network. Therefore, the method for selecting the domain during the terminated call in the related art requires that the MME not only reports the IMS VOIP capability of the EPS network area being accessed by the UE currently, but also indicates the HSS not to select the domain depending on a current IMS VOIP indicator under the circumstance that the ISR is activated, wherein the specific indicating mode can be implemented in various methods. Therefore the MME/SGSN is required not only to send a message to the HSS to indicate the IMS VOIP capability when the IMS VOIP capability of the network area currently being accessed by the UE changes, but also to interact with the HSS and send related information to the HSS each time when the ISR transforms between an active state and an inactive state or each time when ISR is activated. All these requirements aggravate the signaling loads of the network and the processing complexity of the system.
Under the circumstance that a 2G/3G common routing area exists in the network deployment, when the UE which is in the idle state changes between the 2G RAT (radio access technology) and 3G RAT in the common routing area, the routing area update will not be initiated according to the prior art. Therefore, the method for selecting the domain during the terminated call in the related art has, when the method is applied under this circumstance, the problems similar to those when the ISR is activated.